In recent years, a so-called back contact solar cell, in which no electrodes are provided on a light-receiving surface to eliminate optical loss caused by the shadow of electrodes, has been widely studied as a technique of improving the photoelectric conversion efficiency of a crystal silicon solar cell.
FIG. 1 is a schematic diagram depicting an example of the back surface of the back contact solar cell, and FIG. 2 depicts a cross section taken along an alternate long and short dashed line A in FIG. 1. As depicted in FIG. 1, in a solar cell 100, emitter regions (an emitter layer) 112 are formed on the back surface (a first main surface) of a semiconductor substrate (for example, a crystal silicon substrate) 110. Moreover, base regions (a base layer) 113 are formed into the shape of stripes with the emitter regions 112 placed therebetween. Emitter electrodes 122 are formed on the emitter regions 112, and a plurality of the emitter electrodes 122 are coupled by emitter bus bars (bus bar electrode for emitters) 132. Base electrodes 123 are formed on the base regions 113, and a plurality of the base electrodes 123 are coupled by base bus bars (bus bar electrode for bases) 133. On the other hand, the base electrodes 123 are electrically insulated from the emitter regions 112 while the emitter electrodes 122 are electrically insulated from the base regions 113 by insulator films 118. In addition, as depicted in FIG. 2, the solar cell 100 includes passivation films 119 on the first main surface and a second main surface of the semiconductor substrate 110. The passivation films 119 are omitted in FIG. 1.
The structure has been generally formed as follows: after the emitter electrodes 122 and the base electrodes 123 are formed, a resin coating agent is applied to predetermined areas on the substrate by screen printing, inkjet printing, or dispenser application and is completely cured by heat treatment or UV irradiation; and a resin curable conductive paste mainly composed of an electric conductor such as silver, copper, or aluminum is then applied to predetermined areas on the substrate by screen printing, inkjet printing, or dispenser application and is cured by heat treatment.
Patent Document 1 describes a method for forming an electrode by using a polyimide composition for an insulator film, curing the insulator film by heating at 140° C. for 10 minutes and heating at 250° C. for about 30 minutes, then printing silver paste thereon, and performing sintering at 400° C. or higher for 30 seconds.